1. Field of the Invention
The present invention relates to a porous biaxially-oriented film comprising a high molecular ethylene/.alpha.-olefin copolymer. More specifically, the invention relates to a porous biaxially-oriented film having excellent tensile strength and to a separator for batteries for which the porous biaxially-oriented film is preferably used.
2. Description of the Related Art
It has heretofore been known to use a fine porous film of a polypropylene as a separator for batteries since before the filing of the present application as disclosed in, for example, Japanese Patent Publication No. 40119/1971. The film has a fine penetrated porous structure in addition to a rigid nature possessed by the polypropylene, and features rigidity (excellent stiffness), exhibits excellent resistance against mechanical stress at the time of assembling the batteries, is adapted for use as a separator, is easy to use, and permits a lower percent of defects to occur during the production. It has been known that the fine porous film of polypropylene exhibits good electric conductivity near ordinary temperature in a state where an electrolytic solution and an electrolyte are contained in the fine pores but exhibits increased electric resistance in a high temperature region.
In case an abnormal current flows from the battery due to a short-circuit or the like, it can be expected that the separator having such a property works to prevent the flow of an excess current since the temperature rises in the battery due to the thermal resistance and the internal resistance increases. In the case of the polypropylene, however, such a function is not exhibited when the temperature is not higher than 180.degree. C.; i.e., this function cannot be expected at a temperature which is not higher than 150.degree. C.
In recent years, large amounts of energy have been packaged in a small volume owing to the progress in the performance of batteries, and it is quite natural from the standpoint of safety to provide a separator which exhibits a current blocking function when an abnormal current flows even in a relatively low temperature range.
In order to realize such a function (current cut-off function in case of short-circuiting), to reinforce the function and to obtain the function at low temperatures, there has been proposed a separator film for batteries comprising a porous film of a mixture of a polyethylene having a viscosity-average molecular weight of not larger than 300,000 and a polyethylene having a viscosity-average molecular weight of not smaller than 1,000,000 (Japanese Laid-Open Patent Publication No. 21559/1990).
As a related technology, furthermore, Japanese Laid-open Patent Publication No. 105851/1991 discloses a separator for lithium batteries comprising a polyethylene composition which contains an ultra-high molecular polyethylene having a weight-average molecular weight of not smaller than 70,000 in an amount of not smaller than 1% by weight, and has a weight-average molecular weight (Mw)/number-average molecular weight (Mn) ratio of from 10 to 300.
Furthermore, Japanese Laid-Open Patent Publication No. 25305/1993 discloses technology for producing a fine porous polyethylene film used as a separator for batteries, the fine porous polyethylene film being obtained by biaxially orienting a composition comprising 1 to 69% by weight of an ultrahigh molecular polyethylene having a weight-average molecular weight of not smaller than 700,000, 98 to 1% by weight of a high-density polyethylene and 1 to 30% by weight of a low-density polyethylene, and having a weight-average molecular weight/number-average molecular weight ratio of from 10 to 300, or by biaxially stretching a composition comprising 30 to 90% by weight of the above-mentioned ultrahigh molecular polyethylene and 70 to 10% by weight of a low-density polyethylene.
When compared with the conventional fine porous polypropylene films, the above-mentioned films have succeeded in imparting a current cut-off function at a low temperature (e.g., 130.degree. C.).
The basic idea of the prior art technology is to accomplish the cut-off of current at a low temperature by biaxially orienting the ultrahigh molecular polyethylene to maintain the film strength, and by adding a high-density polyethylene or a low-density polyethylene having widely ranging molecular weight distribution to form a structure which is softened or is melted at a low temperature.
It has been considered that the electric resistance of the separator increases as the fine pores are clogged. This phenomenon can be represented by when a polyethylene having wide range of molecular weight distribution and a polyethylene having a different molecular weight are blended, the polymer gradually melts over a wide range of temperatures. Therefore, obtaining a current cut-off function at a low temperature results in an increase in the internal resistance in a practicable range of, for example, about 100.degree. C., which is not desirable from the standpoint of the battery.
Broadening of the distribution of the molecular weight is well used to increase the flowability at the time of molding high-molecular-weight polyethylene. As is understood from this fact, a film obtainable from a blend of broad molecular weight distribution polyethylenes or polyethylenes which become highly flowable after the beginning of melting is excellent because it enables shutting off of a current within a short period of time. But when thereafter, for some reason, the temperature is further elevated, since the film is highly flowable, the film which has been once closed has a risk of breaking.
That is, as far as a composition containing a polyethylene having broad range of molecular weight distribution is relied upon, it becomes necessary to manage to stop the battery function within a relatively narrow temperature range between a temperature at which the separator loses gas permeating property as the temperature of the battery rises and a temperature at which the separator is fluidized and breaks.
In order to reliably accomplish this, the internal resistance of the battery must have been increased in advance by suppressing gas permeating property of the separator sacrificing, however, the discharge performance of the battery.
Ideally, therefore, it is desired that the polyethylene separator suddenly loses gas permeating property when a maximum practicable temperature is exceeded, but is not fluidized and does not break even after it is melted. That is, by using a porous film obtained from a composition of polyethylenes, it is not possible at present to lower the temperature at which the porous film loses gas permeating property and to prevent the porous film from fluidizing and breaking after it is once melted.